Nitronyl Nitroxide Biradicals Research Articles

Supramolecular heptanuclear Ln-Cu complexes involving nitronyl nitroxide biradicals: structure and magnetic behavior.

Four novel heptanuclear Ln-Cu complexes with the formula [Ln2Cu(hfac)8(NITPhTzbis)2][LnCu(hfac)5(NITPhTzbis)]2 (LnCu = YCu 1, TbCu 2, DyCu 3 and HoCu 4; hfac = hexafluoroacetylacetonate) were successfully constructed by employing the triazole functionalized nitronyl nitroxide biradical ligand NITPh-Tzbis (NITPh-Tzbis = 5-(1,2,4-triazolyl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). These hetero-tri-spin complexes are composed of two biradical-bridged dinuclear [(LnCu(hfac)5(NITPhTzbis)] units and one trinuclear [Ln2Cu(hfac)8(NITPhTzbis)2] unit which form a heptanuclear supramolecular structure through π-π interactions. Magnetic susceptibility investigations indicate that ferromagnetic exchange interactions dominate at low temperature for this supramolecular system which can be attributed to the Ln-nitroxide exchange and intramolecular NIT⋯NIT coupling mediated by the m-phenylene moiety. The DyCu derivative was found to exhibit a slow magnetic relaxation behavior.

Syntheses, structures, and magnetic properties of the lanthanide complexes of imidazole-substituted nitronyl nitroxide biradicals

Two series of lanthanide complexes of two imidazole-substituted nitronyl nitroxide biradicals have been synthesized and characterized structurally and magnetically.

Magnetic Relaxation in a Dysprosium–Copper Heterometallic Cluster Involving Nitronyl Nitroxide Biradicals

Magnetic Relaxation in a Dysprosium–Copper Heterometallic Cluster Involving Nitronyl Nitroxide Biradicals

Regulating Spin Dynamics of Nitronyl Nitroxide Biradical Lanthanide Complexes through Introducing Different Transition Metals.

Four biradical-Ln complexes with different transition metal ions, namely [LnM(hfac)5 (NITPh-PyPzbis)] (MII =MnII and LnIII =Gd 1, Dy 2; MII =NiII and LnIII =Tb 3, Dy 4), were prepared by the reaction of Ln(hfac)3 ⋅ 2H2 O, Mn(hfac)2 ⋅ 2H2 O or Ni(hfac)2 ⋅ 2H2 O with NITPh-PyPzbis biradical (hfac=hexafluoroacetylacetonate, NITPh-PyPzbis=5-(3-(2-pyridinyl)-1H-pyrazol-1-yl)-1,3-bis(1'-oxyl-3'-oxido- 4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene). In complexes 1-4, the NITPh-PyPzbis biradical chelates one LnIII ion by means of its aminoxyl moieties and the transition metal ion is introduced through the two N donors from the pyridyl pyrazolyl moiety. Magnetic investigations indicate that complex 4 displays visible maxima in frequency/temperature-dependent χ'' signals with two-step relaxation processes, but complex 2 exhibits no slow magnetization relaxation. The comparison of structure parameters of both Dy complexes indicates that the symmetries of coordination spheres of two Dy ions are D2d for 2 and C2v for 4, which thus probably results in different magnetic relaxation behaviors. This work provides new insight for improving properties of Ln-biradical based SMMs.

Nitronyl Nitroxide Bifunctionalized Electron-Poor Chromophores: Synthesis of Stable Dye Biradicals by Lewis Acid Promoted Desilylation.

Open shell organic molecules bearing π-cores are of great interest for optical, electronic, and magnetic applications but frequently suffer fast decomposition or lack synthetic accessibility. In this regard, nitronyl nitroxides are promising candidates for stable (bi-)radicals due to their high degree of spin delocalization along the O-N-C-N-O pentad unit. Unfortunately, they are limited to electron-rich systems so far. To overcome this limitation, we developed a synthetic procedure for the twofold spin decoration of electron-poor chromophores (Ered = -1158 mV) with nitronyl nitroxide radical moieties via selective deprotection/oxidation of the respective silylated precursors with boron fluoride and subsequent quenching with tetraethyl orthosilicate. Nitronyl nitroxide biradicals PBI-NN, IIn-NN, PhDPP-NN, ThDPP-NN, and FuDPP-NN bridged by perylene bisimide (PBI), isoindigo (IIn), and diketopyrrolopyrrole (DPP) pigment colorants were finally obtained as bench stable compounds after periodate oxidation with yields of 60-81%. The absorption spectral signatures of the chromophores remain preserved in the open shell state and match the ones of the pristine parent compounds, which allowed an a priori prediction of their optical properties. Consequently, we achieved twofold spin labeling while keeping the intrinsic properties of the electron deficient chromophores intact.

Slow magnetic relaxation in CoII-LnIII heterodinuclear complexes achieved through a functionalized nitronyl nitroxide biradical.

A new nitronyl nitroxide biradical ligand NITPh-PyPzbis (5-(3-(2-pyridinyl)-1H-pyrazol-1-yl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)benzene) has been successfully applied for constructing a 3d-4f CoII-LnIII system, giving rise to a family of novel hetero-tri-spin complexes, namely, [LnCo(hfac)5(NITPh-PyPzbis)]·CH2Cl2 (LnCo = YCo 1, GdCo 2, TbCo 3, DyCo 4, and HoCo 5; hfac = hexafluoroacetylacetonate). In these hetero-tri-spin complexes, the NITPh-PyPzbis biradical chelates one LnIII and one CoII simultaneously by means of its adjacent aminoxyl moieties and two N donors from the 3-(2-pyridinyl)-1H-pyrazol-1-yl unit, respectively, realizing the unique biradical-Co-Ln heterodinuclear structure. Direct-current magnetic susceptibility investigations show that antiferromagnetic coupling is predominant in the YCo derivative, while the leading magnetic interaction in the GdCo analogue is ferromagnetic. Alternating-current data for the DyCo complex display visible temperature/frequency-dependent χ'' peaks, indicating the SMM behavior. In the fluorescence spectra, four characteristic emission bands of the TbIII ion have been detected for the TbCo analogue.

Heterobimetallic MnII2DyIII2 cluster involving nitronyl nitroxide biradicals: Structure and magnetic properties

A novel nitronyl nitroxide biradical-based MnII2DyIII2 compound [Dy2Mn2(hfac)10(NITPhPybis)2] has been designed and assembled successfully, representing the first example of nitronyl nitroxide-MnII-LnIII compounds and expanding the realm of radical-metal based magnetic materials. DC magnetic susceptibility studies reveal the existence of ferromagnetic Dy-nitronyl nitroxide interaction and strong antiferromagnetic interaction between direct coordinated NO group and Mn(II) ion.

Functionalized Nitronyl Nitroxide Biradicals for the Construction of 3d-4f Heterometallic Compounds.

Functionalized nitronyl nitroxide biradical ligands incorporating pyridine groups hold CoII and LnIII ions together, creating biradical-based 3d-4f tetranuclear complexes [Ln2Co2(hfac)10(NITPhPybis)2] [LnIII = Gd (1), Tb (2), Dy (3), and Ho (4); NITPhPybis = 5-(4-pyridyl)-1,3-bis(1'-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1 H-imidazol-2-yl)benzene; hfac = hexafluoroacetylacetonate]. These complexes have a centrosymmetric cyclic molecular structure in which two biradicals perform as tetradentate ligands to bind two CoII and two LnIII ions, resulting in a rare octaspin system. Direct-current (dc) magnetic susceptibility studies reveal that the strong antiferromagnetic CoII-NO magnetic exchange dominates the present magnetic system, while magnetic coupling of Gd-ON is ferromagnetic. Analysis of the magnetic data of the Gd complex allows us to determine the magnetic parameters through the appropriate magnetic model. Alternating-current (ac) magnetic susceptibility investigations indicate that 2 displays frequency-dependent out-of-phase signals under a zero dc field, while ac magnetic susceptibilities of 3 show field-induced frequency dependence, which is a typical feature of slow relaxation of the magnetization. Complexes 1-4 represent the first nitronyl nitroxide biradical-based 3d-4f compounds.

Single-molecule magnet behavior in a CuII-decorated {DyIII2} complex with nitronyl nitroxide biradicals

Unprecedented octa-spin-center 2p–3d–4f compounds were obtained using a specially designed nitronyl nitroxide biradical, where the Dy analogue exhibited SMM behavior.

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides.

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through p-ter-phenyl (PPP), ter-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of 3, 5, 6, PPP-NNSi, PTP-NNSi, and final biradicals of TTT-NN, TPT-NN, and PPP-NN, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in TTT-NN and TPT-NN than for NN and phenyl units (∼29°) in PPP-NN were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals TTT-NN, TPT-NN, PTP-NN, and PPP-NN were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for TTT-NN and TPT-NN. The intramolecular exchange interactions for TPT-NN and TTT-NN were found in good agreement with the ones calculated by broken symmetry DFT calculations.

Rigid Core Anthracene and Anthraquinone Linked Nitronyl and Iminoyl Nitroxide Biradicals

The first syntheses of bis(nitronyl nitroxide) and bis(iminoyl nitroxide) (diNN, diIN) biradicals linked through rigid acene core conjugating anthracene (A) and anthraquinone (AQ) units are reported. Computational modeling predicts weak intramolecular exchange in AQ-linked systems, but A-linked biradicals to have ground state multiplicities consistent with the Borden-Davidson disjointness model. Solution electron spin resonance spectra showed inter-radical exchange-coupled triplet states, except for 2,6-AQ biradicals showing isolated spin spectra. Crystallography of the A-linked biradicals shows a key role for inter-radical contacts for molecular packing. DiINs showed lower-dimensional dyad packing with disorder at the radical units: the conformationally more symmetrical diNNs gave staircase one-dimensional or brickwork two-dimensional lattices. Core anthracene unit stacking was only seen in two systems with bromine on the central anthracene ring: the (large) bromine occupies alternate side placement in d...

Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals: Candidates for Quantum Magnets

The tolane bridged nitronyl nitroxide biradicals with hydrogen bond donor and acceptor functional groups were synthesized. The magnetic measurements and the DFT calculations were performed to ascertain the influence of the functional groups on inter- and intramolecular magnetic exchange interactions. While upon functionalizing the tolane bridge the intramolecular exchange interactions remained nearly unchanged, the fine-tuning of intermolecular exchange interactions could be achieved by employing the crystal engineering approach.

Interacting networks of purely organic spin–1/2 dimers

The fine-tuning of the intra-molecular exchange interactions in π-conjugated nitronyl nitroxide biradicals and their crystalline network formation is described.

ChemInform Abstract: Synthesis of Stable Free Radicals: A Novel Family of Oligopyridine Based Nitronyl-Nitroxide Biradicals.

Abstract Diformyl substituted pyridines (at the 2,5 and 2,6 positions), 2,2′-bipyridines (at the 4,4′, 5,5′, and 6,6′ positions), 3,6-di-(6-carboxyaldehyde-2-pyridyl)-pyridazine and 2,9-dicarboxyaldehyde-1,10-phenanthroline react with N,N′-dihydroxy-2,3-diamino-2,3-dimethylbutane to yield the bis-N,N′-dihydroxyimidazolidine. Mild oxidation afforded the corresponding nitronyl-nitroxide biradicals 1 to 7. These ligands are highly colored and form very stable transition metal complexes.

Coupled spin S = 1/2 dimer systems based on nitronyl-nitroxide biradicals

Due to the collective behaviour of their magnetic excitations, arrays of coupled S = 1/2 spin dimers, exposed to a magnetic field, are of general interest in solid state physics as they can be considered as the source for generating a gas of interacting bosons. We present a new class of coupled spin-dimer systems based on purely organic building blocks. From magnetic susceptibility and magnetization data of various nitronyl-nitroxide biradicals, we extract the influence of the bridging ligands on the intra-dimer coupling constant Jdimer. The target materials show moderate values of Jdimer ranging from 5 to 10 K, and indications for significant inter-dimer couplings. The moderate size of these couplings, together with the high flexibility against chemical modifications, make these materials promising candidates for studying interacting quantum objects under well-controlled conditions.

Shift of stereochemical nonrigidity from coordination units to polymethylene fragments in heterospin copper(II) hexafluoroacetylacetonate complexes with nitronyl nitroxide biradicals

The reactions of bis(hexafluoroacetylacetonato)copper(II) [Cu(hfac)2] with the nitronyl nitroxide biradicals bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]alkanes (L6, L10, and L12) produced the framework heterospin complex [Cu(hfac)2]2L6 and the layer-polymeric heterospin complexes [Cu(hfac)2]2L10 and {[Cu(hfac)2]2L12} [Cu(hfac)2(PriOH)2], respectively. In the solid state of these compounds, the stereochemical nonrigidity is manifested as a deformation of the polymethylene fragments-(CH2)n-.

Ground-State Triplet Biradicals of Nitronyl Nitroxide Containing a Nucleobase Substituent as Synthons for Bio-Inspired Organic Magnets

An organic nitronyl nitroxide biradical substituted with a nucleobase of cytosine was designed and synthesized. The molecular ground state of the biradical was found to be triplet (S=1) with a singlet-triplet energy gap of 2J/kB=50.6 K from magnetic susceptibility measurements. The ground-state triplet biradical serves as an S=1 building block for bio-inspired moleculebased magnets with hydrogen-bonded nucleobase pairings.

Magnetic interactions in p-phenylene-bis(nitronyl nitroxide) biradicals with large torsion angles

We have designed and synthesized new biradicals of p-phenylene-bis(nitronyl nitroxide) substituted with two methoxy groups at 2,3- ( 2) and 2,5-positions ( 3). A parent biradical p-phenylene-bis(nitronyl nitroxide) ( 1) has intramolecular antiferromagnetic exchange interaction of 2 J/ k B = −104 K ∼ −106 K with a torsion angle of 28.5° between the phenyl and the imidazole rings of nitronyl nitroxide. X-ray crystal structure analysis shows that the bulky substituents in 2 and 3 give large torsion angles of 65–70°. The larger torsion angles should weaken the magnitude of intramolecular exchange interactions, which is attributed to a decrease in π-conjugation over the p-phenylene and the radical groups. Magnetic susceptibility measurements indicate that the intramolecular exchange interactions in 2 and 3 are severely weakened to about 6% of that in 1, 2 J/ k B = −6 K ∼ −8 K. The relation between the torsion angle and the intramolecular exchange interaction is consistent with DFT calculations. The ground-state singlet biradicals with suppressed intramolecular exchange interactions can be a building block for exotic exchange-coupled spin systems as predicted in our theoretical studies.

2D and 3D Cu(hfac)2 Complexes with Nitronyl Nitroxide Biradicals

Reactions between Cu(hfac)2 and nitronyl nitroxide biradicals 1,4-bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]butane (L4) and 1,8-bis[4-(4,4,5,5-tetramethyl-3-oxide-1-oxyl-4,5-dihydro-1H-imidazol-2-yl)pyrazol-1-yl]octane (L8) gave respectively a framework compound [Cu(hfac)2]2L4 and a layered polymer compound [Cu(hfac)2]2L8. The framework of [Cu(hfac)2]2L4 consists of 66-membered condensed metallocycles. Inside the framework, the structure has macrohelixes (pitch approximately 25 A) extending along the [001] crystallographic direction. All the helixes have the same direction of winding; the crystals, therefore, are optically active, the structure corresponding either to P-isomer (P4(1)2(1)2) or to M-isomer (P4(3)2(1)2). The long distances between the Cu atoms and the O atoms of the coordinated >N-O groups (Cu-O 2.351-2.467 A) are responsible for ferromagnetic exchange interactions in Cu2+-O-N< and >N-O-Cu2+-O-N< exchange clusters.

Pyridine-substituted nitronyl nitroxide biradicals: a triplet (S = 1) ground state lasting out N-methylation

A 3,5-pyridine-substituted biradical of nitronyl nitroxide (1) has been designed, synthesized and the magnetic properties fully characterized. The ground-state spin multiplicity of 1 has been found to be triplet (S = 1) with a singlet–triplet energy gap of 2J/kB = 40 K from magnetic susceptibility measurements on a magnetically diluted system dispersed in organic polymer films. The 3,5-substituted pyridine 1 has a hydrogen-accepting site which is more accessible to hydrogen donors than previously known biradicals with sterically hindered 2,6-pyridine frameworks. N-Methylation of 1 has yielded a stable cationic species in a trifluoromethanesulfonate salt (2+·TfO−). The ground state of the cation 2+ has been found to be triplet as well with 2J/kB = 32 K from magnetic susceptibility measurements for magnetically diluted films. The magnetic susceptibility of neat crystalline solids of 1 and 2+·TfO− has been explained by Heisenberg exchange coupling models based on their X-ray crystal structures. It is well known that the energy preference of a high-spin ground state for m-phenylene, or m-xylylene, coupling units is disturbed in such cases as the π-conjugation is affected by heteroatomic substitution, an ionic charge, or molecular conformation. The present experimental results show that the high-spin preference in 1 and 2+ is little influenced by the heterocycle or the ionic charge. Intermolecular noncovalent bonds such as hydrogen bonding and electrostatic interactions are a driving force for crystallization of open-shell molecules in a controllable manner. The ground-state triplet biradicals serve as building blocks for molecule-based magnets of S > 1/2 based on intermolecular noncovalent bonding architecture.